September and October are when we have our worst fires, so it is a good time to review what science tells us about protecting our homes.

One thing we need to know is that wildfire does not move like an avalanche with momentum that envelops everything in its path. Fire is a chemical process that takes place only when conditions for combustion are present. Fire requires the right amount of oxygen, heat, and fuel. Reduce any of these three significantly and the fire will slow or stop. This is why some houses on the same street will burn and others will not. Some are more vulnerable, or more ignitable, than others.

We should know that the terrifying "crown fire", which races through the canopy of the forest with huge flame lengths, has its limits. Scientists conducted experiments in a Canadian forest to see from what distance the radiant heat from a crown fire could ignite the wood that is found on many house exterior walls. The answer was encouraging-fire has to be fairly close to wooden siding to set in on fire, generally closer than 66 feet. Wood closer than 66 feet that ignited could not sustain the fire when the flame front passed and self-extinguished. It is unlikely that a crown fire over a hundred feet away from a house will ignite it from radiant heat.

USFS scientist Jack D. Cohen created a model to show how ignitions take place across open space given a 66 foot wall of flame. His model shows, for instance, that it would take 10 minutes of continuous heat to set wood on fire at a distance of 131 feet. The good news in this is that most wildfire flame fronts pass in about a minute. (This flame front is lethal to humans, so we should still evacuate. Our skin and lungs will be damaged long before wood ignites.)

So if the flame front does not burn most houses, what does? Case studies of actual home losses from western fires have shown that most homes are destroyed by the ignition caused by flying embers or surface fuels (duff) that lead up to the house. Fires can throw hot embers over a mile ahead of their front. Even after the front passes, these embers can lie on ignitable portions of a house and slowly set it on fire. Wood shingle roofs are extremely vulnerable to this attack.

Another problem is called "ember intrusion", where winds whip embers into attics, crawl spaces, or open windows. In the 2008 Freeway Fire that started in the Santa Ana River bed and raced into Orange County, many houses were lost when winds threw embers underneath otherwise excellent fire resistant tile roofs. The solution to both of these problems are "bird stops" that plug the ends of tile runs, smaller vent (1/8") screens, and newly designed ember catching vents.

Still another common cause of home loss was staggeringly simple: low intensity flames just crept up to the house along a path of needles and leaves and set it on fire. We all know the solution to this (a rake), but we should never forget how dangerous lowly needles can be.

If you want to learn more about making your house less ignitable, our project managers (659-6208) will be happy to discuss it with you.